1. Field of the Invention
The present invention relates to a method for manufacturing a coil spring which has a predetermined free length and also has a desirable elasticity.
2. Prior Art
Conventionally, as shown in FIG. 4, coil springs are manufactured using a coiling pin 1, a pitch tool 2, etc. incorporated in a coil fabricating apparatus.
A wire material W, which is formed into a coil spring, is passed through a wire guide 3 and fed out by being sandwiched between a pair of feed rollers 4 and 5. The wire material W is further passed through wire guides 6 and 6a and is caused to contact the coiling pin 1 so that the wire material W is shifted onto a core piece 7.
The wire material W is thus formed into a helical shape of a prescribed pitch by the pitch tool 2. The helical shaped wire material W is then cut by a cutting tool 8 into a coil spring.
Generally, coil springs are required to have a predetermined free length and a desirable performance; and it is particularly necessary that a predetermined free length, which is the total length of a coil spring, is constant for each and every coil spring manufactured.
Conventionally, a single coil spring is manufactured by feeding a wire material which has a length that is necessary to fabricate a single coil spring, and then the free length of the completed coil spring is measured by a contact or non-contact type sensor. The free length is compared with a predetermined set-length, and coil springs which are longer or shorter than the reference length are discarded as defective products. If the defective products exceed a certain number, a motor which adjusts the pitch tool 2 is actuated so as to finely adjust the pitch, thus insuring that subsequent coil springs will have the predetermined free length.
Usually, the free length of a coil spring is affected by the characteristics of the wire material itself and by the variation in the wire habit, tensile force, etc. of the wire material. Thus, some coil fabricating apparatuses take such factors into consideration in order to manufacture coils which have a predetermined free length.
However, in these systems, since the free length of the finished coil spring is checked after the completion of wire fabrication to find satisfactory and defective springs, there still are problems. In particular, the coil springs are measured, after being made, for its free length, but obviously the finished springs cannot be modified. In addition, the wire material includes factors (such as wire habit, etc.) which can be altered during the process of wire pulling which is one of the steps of coil spring manufacturing. Such factors can greatly affect the resulted springs. Accordingly, even if the wire material has a predetermined length, the free length of the coil spring can vary and does not sustain consistency.
Furthermore, in the systems which take the wire material characteristics and the variations in wire habit, tensile force, etc. into account, such elements are brought into the tool set-up process or into the reference values input process, which are performed in the initial stage of coil manufacturing. If the material factors change during the manufacturing process, since the initial set-ups as described above cannot be altered in response to these changes, the number of defective products tends to be high when the systems where the free length is measured after the completion of coiling is utilized. In particular, when an attempt is made to produce coil springs with a highly precise free length, the rate of satisfactory products tends to drop.